1. Field of the Invention
The invention relates to a micromachined inductive sensor for detecting, without contact and through the intermediary of a radiated alternating magnetic field, the position and/or the movement of an object capable of modifying this magnetic field.
More particularly, the invention relates to a sensor of this type comprising at least one primary inductive coil and at least one secondary detecting coil of planar design, this term meaning that the thickness of the windings of the coils has a value substantially lower than the other dimensions of the coils and that the coils are disposed on a flat support
2. Description of the prior art
Sensors comprising planar coils have the advantage that they can be produced by techniques similar to those normally employed for the production of integrated circuits with which they can also advantageously be associated on the same silicon wafer, the integrated circuit then being in particular formed by the circuits for processing the measurement signal provided by the sensor.
Such a sensor was described in a patent application filed in Switzerland on Apr. 29, 1996 in the name of the Applicant and having the title: "Device for detecting position and movement using magnetic field variation".
FIG. 1 of the appended drawings represents by way of reminder a very simplified diagram in perspective view of a sensor according to this patent application. It comprises a spirally wound planar transmitter sensor 1 which is connected to an alternating voltage source S transmitting a current into this coil, the coil 1 thus transmitting an inductive magnetic flux B1 perpendicular to the plane P1 of the coil 1.
The sensor also includes two planar receiver coils 2 and 3 also spirally wound and located in a plane P2 in the sensor with respect to the transmitter coil 1 in such a way that they both receive a determined portion of the magnetic flux B1 generated by the transmitter coil 1. These portions of magnetic flux are denoted respectively B2 and B3 on FIG. 1.
The receiver coils 2 and 3 are connected to an operating circuit CE the function of which is to process, preferably differentially, the measurement signals obtained through the intermediary of the two receiver coils. A detailed description of this circuit can be found in the patent application cited above.
The sensor is capable of detecting the position and/or the movement of an object O which, because of its structure and the material from which it is made, is capable of modifying the flux induction conditions in the two receiver coils 2 and 3. To this end, the object O has a repetitive or non-repetitive structural discontinuity which, when it passes in front of the sensor, makes it possible to obtain the changes in the induction conditions. In the example in FIG. 1, this structural discontinuity is formed by a series of successive teeth D and notches C provided on the edge of the object O. In the example shown, the general plane of the object O is parallel to the planes of the coils 1 and 2 and it can be assumed that the object O is a toothed wheel for example, driven in rotation around an axis which is disposed parallel to the axes of the coils 1 to 3. However, the mutual attitudes of the sensor and the object O are not determining factors provided that the structural discontinuity of the object O can modify the induction conditions which, to simplify, will be the case if a certain distance between the sensor and the object O is respected for a given intensity of the inductive current.
Since the induction current is alternating, for example a sinusoidal current of determined frequency, alternating voltages are induced in the receiver coils 2 and 3. These voltages will have amplitudes determined by the relative positions of the sensor and the object; the amplitudes will be constant if the object O is not in movement. However, if this object O is moved, for example under the impulsion of a rotation movement around its axis, the voltages induced will be modulated by the movement in front of the sensor of the structural discontinuity of the object, and this modulation can be used in the circuit CE to extract information signals concerning movement of the object O. It will be understood that the signals of the different receiver coils, which are offset with respect to a given discontinuity, can serve at any time to determine the speed, distance, acceleration or deceleration, direction of movement or position of the object O.
Since this sensor works through inductive coupling, it can be compared to a transformer in which the transmitter coil 1 is the primary and in which the receiver coils 2 and 3 are the secondaries. In the earlier example described hereinabove, this transformer comprises no flux guiding elements. However, such an element can be provided for example in the form of a layer of ferromagnetic material that can be disposed above the coils 1 and 3, for example.
This "transforming" sensor can work with objects O made of a magnetic material or, also, of a non-magnetic but electrically conductive material in which the magnetic field of the transmitter coil 1 will be capable of generating eddy currents in turn disturbing the magnetic fluxes of the receiver coils 2 and 3, such disturbances finally leading to the required signal being obtained in the operating circuit CE.
The development of the sensor thus designed by the Applicant led to the finding that the coils are inter-coupled not only magnetically, but also capacitively. This is not surprising in itself, because capacitive coupling phenomena are found in any system formed from magnetically coupled coils. In themselves, these phenomena have little effect in sensor systems of macroscopic size when the influence of undesired capacitances on the desired signal hardly exceed 1%, in general.
However, in the sensors concerned here and which are of microscopic size (all the coils described hereinabove can have overall dimensions in the order of only a few millimeters), undesired capacitances can cause undesired influences the extent of which can be up to 100% of the desired signal, if it is also taken into consideration that the working frequency of the sensor is relatively high (in the order of 1 MHz, for example).
FIG. 2 of the annexed drawings shows an equivalent simplified diagram of the sensor that has just been described, the magnetic coupling being symbolized by the double arrow M.
This diagram shows that each coil 1, 2 and 3 is in fact associated with a parallel undesired capacitance respectively Cp1 to Cp3. These capacitances certainly have a harmful influence, but they can be canceled out or at least taken into account in the current source S, on the one hand, and in the operating circuit CE, on the other hand. However, it is found that other undesired capacitances affect the behavior of the sensor. The coils 1 to 3 are intercoupled not only magnetically, but also capacitively, shown in FIG. 2 by the capacitances Cc1 to Cc4, connecting each end of a receiver coil to the transmitter coil, as represented.
Unlike the parallel capacitances Cp1 to Cp3, the coupling capacitances Cc1 to Cc4 between the coils 1 to 3 are not generally equal and can be canceled out or eliminated electronically only with means of a complexity that would almost nullify the advantages of simplicity of construction that can be obtained with the sensor described hereinabove. In addition, these undesired coupling capacitances unfavorably affect the result of the measurement that the sensor may be expected to obtain. First of all, they contribute to the signals applied by the receiver coils 2 and 3 through components that are in phase opposition with these desired signals such that these components are likely to cancel completely the desired signals in coils 2 and 3. If however, which is practically always the case however much care is taken in manufacture, the undesired capacitances are not equal for the two receiver coils, they destroy the differential effect that the coils have on the measurement signal.